1. Field of the Invention
The present invention relates to air moving devices and, in particular, to centrifugal blowers which include impellers or fan wheels having forward curved blades that are used, for example, in modern gas furnace draft inducer applications.
2. Description of the Related Art
In high efficiency furnaces, standard chimney air-draw effects are not sufficient to assure the required air flow through the furnace heat exchangers, and therefore, modern high efficiency furnaces utilize draft inducer blowers to provide sufficient air flow through the heat exchangers of the furnace. These types of draft inducer blowers typically include impellers or fan wheels having forward curved blades. The impeller is rotated in a scroll shaped blower housing to draw an air flow through the housing. This, in turn, draws an air flow through the heat exchanger. Similarly, in other applications where air flow is produced by a centrifugal blower having forward curved blades, the ability of the blower to efficiently generate sufficient air flow and pressure are important. Also, in many applications in which centrifugal blowers are used, such as furnace draft inducers, for example, space is at a premium so minimization of the size of the blower is desired.
Centrifugal blowers convert static air pressure into velocity air pressure in the blower housing. Pressure conversion is accomplished in the blower housing as the cross section available for passage of the air flow expands around the periphery of the impeller from the cutoff to the outlet. FIG. 1 is a schematic representation of a typical prior art blower housing and impeller, and a graph showing the dimensional relationship of the impeller periphery IP and the scroll shaped length of the blower housing side wall SS. As shown in FIG. 1, the increase in cross section in the scroll portion of the blower housing around the impeller is proportional to the developed length of the impeller periphery. In particular, the angle between the developed scroll surface SS and the impeller periphery IP is called the expansion angle which, as shown in FIG. 1, is 7°. The impeller diameter and the expansion angle determine the overall width dimensions W1-W1 and W2-W2of the scroll length of the blower housing.
The effect of expansion angle on blower performance is shown in the pressure-flow curves in FIG. 2. The curves in FIG. 2 represent blower housing side walls having expansion angles of 4, 6, 8, 10, and 12 degrees. Flow rate increases significantly with increases in expansion angle at any constant static pressure between free flow (zero static pressure) at the bottom of each pressure-flow curve and the knee of the curve at the top. For example, at a static pressure of 30% of maximum, the air flow rate is only 40% of maximum for a 4° expansion angle but is 90% for a 10° expansion angle.
Expansion angle also effects performance of the blower in a particular system. As shown in FIG. 2, for example, the impeller in a blower housing having an 8° expansion angle delivers about 73% of the free flow air rate at operating point A on the given system resistance curve. If the expansion angle of the blower housing is increased to 10°, for a constant expansion angle scroll housing air delivery of the same impeller is increased to about 83% of free flow air at operating point B.
Although greater expansion angles improve blower performance, the relative amount of improvement gradually diminishes, and the size of the blower housing with respect to the diameter of the impeller becomes too large for space constraints in applications in which the blower is used. This is mostly due to the volume between the impeller periphery and the blower housing side wall too great to allow the high velocity stream coming off of the impeller to impact the air volume in the scroll. For example, if either of the overall width dimensions W1-W1 or W2-W2 of the blower housing is too large for the space available for the blower housing, a blower housing having a smaller expansion angle may be selected. Then, if the resulting reduction in air flow rate is not acceptable, a compromise must be made in either blower size or air performance.
One known blower assembly 10 is shown in FIGS. 3 and 4. This assembly 10 generally includes a blower housing 12 having a top wall or end wall 14 and a side wall 16 extending from top wall 14. The side wall 16 includes a flange 18 by which a cover member (not shown) may be secured to the side wall 16 such as by crimping or welding. The cover member typically includes a circular inlet opening (not shown). A motor 20 is attached to top wall 14 of blower housing 12 via suitable fasteners (not shown). An impeller 22 is attached to output shaft 24 of motor 20 and is positioned within the interior of blower housing 12. The impeller 22 is a “fan wheel,” “squirrel cage” or “sirocco” type impeller, including a plurality of blades 26 which are curved forward with respect to the direction of air flow, indicated by arrow 28. Side wall 16 of blower housing 12 is generally curved or scrolled as described below, and defines a rectangular air outlet opening 30 to which a typical discharge structure (not shown) may be attached, for example, for connection to a circular discharge pipe via suitable clamps and/or fasteners. Cutoff 32 is defined by the end of the scroll shaped side wall 16 adjacent outlet opening 30.
As shown in FIG. 4, the output shaft 24 of the motor 20 and the center of impeller 22 are coaxial and disposed at a center point CP. Side wall 16 of blower housing 12 is scrolled such that its radius R1, defined from center point CP to wall 16, continuously increases in length from cutoff 32 in a radial direction center point CP with respect to the direction of rotation of impeller 22 and the air flow direction along arrow 28. Thus, radius R1 has a minimum length at cutoff 32 and a maximum length adjacent the end of the outlet opening 30 which is the cutoff 32.
In this manner, the side wall 16 of blower housing 12 is shaped to provide the blower housing 12 with a constantly expanding internal area between the impeller 22 and the side wall 16 around impeller 22 from the cutoff 32 toward the outlet opening 30 in order to allow constant expansion of the air flow area from impeller 22 toward outlet 30. However, in view of the considerations discussed above, the expansion angle of the blower housing 12 is typically only about 6° or less in order to minimize the overall width dimensions W1-W1 and W2-W2 of the blower housing.
What is needed is a blower housing which is an improvement over the foregoing.